Fix crash on marking all updates

[HebaruSan]

Problem

0. Have some upgradeable mods (you can install old versions in the Versions tab for testing)
1. Switch to the "Newly compatible filter" when it's empty (i.e., nothing is newly compatible)
2. Click "Add available updates"

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
  at System.Collections.ArrayList.get_Item (System.Int32 index) [0x0000d] in <a5e6a8b38be74ec1b9abd482cb076c8a>:0 
  at System.Windows.Forms.DataGridViewRowCollection.get_Item (System.Int32 index) [0x00000] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at CKAN.ManageMods.MarkAllUpdates () [0x000fb] in <1e9e6cc7a1124c26a7e978a1dd44c3d6>:0 
  at CKAN.ManageMods.MarkAllUpdatesToolButton_Click (System.Object sender, System.EventArgs e) [0x00001] in <1e9e6cc7a1124c26a7e978a1dd44c3d6>:0 
  at System.Windows.Forms.ToolStripItem.OnClick (System.EventArgs e) [0x00019] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.ToolStripMenuItem.OnClick (System.EventArgs e) [0x00090] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.ToolStripMenuItem.HandleClick (System.Int32 mouse_clicks, System.EventArgs e) [0x00000] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.ToolStripItem.FireEvent (System.EventArgs e, System.Windows.Forms.ToolStripItemEventType met) [0x00054] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at (wrapper remoting-invoke-with-check) System.Windows.Forms.ToolStripItem.FireEvent(System.EventArgs,System.Windows.Forms.ToolStripItemEventType)
  at System.Windows.Forms.ToolStrip.OnMouseUp (System.Windows.Forms.MouseEventArgs mea) [0x00048] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.Control.WmLButtonUp (System.Windows.Forms.Message& m) [0x00078] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.Control.WndProc (System.Windows.Forms.Message& m) [0x001b4] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.ScrollableControl.WndProc (System.Windows.Forms.Message& m) [0x00000] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.ToolStrip.WndProc (System.Windows.Forms.Message& m) [0x00000] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.MenuStrip.WndProc (System.Windows.Forms.Message& m) [0x00000] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.Control+ControlWindowTarget.OnMessage (System.Windows.Forms.Message& m) [0x00000] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.Control+ControlNativeWindow.WndProc (System.Windows.Forms.Message& m) [0x0000b] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
  at System.Windows.Forms.NativeWindow.WndProc (System.IntPtr hWnd, System.Windows.Forms.Msg msg, System.IntPtr wParam, System.IntPtr lParam) [0x00085] in <5e01e3d59bfc48e1a1ea0509bb585d97>:0 
exception inside UnhandledException handler: (null) assembly:/media/DataTrove/mono-dev/lib/mono/4.5/mscorlib.dll type:InvalidOperationException member:(null)

Even if you have a few rows to avoid the crash, only updates with rows visible in the current filter will be marked.

Cause

ModGrid.Rows is empty when this line tries to access its first element:

CKAN/GUI/Controls/ManageMods.cs

Lines 404 to 405 in cfb5881

	// Select the top row and scroll the list to it.
	ModGrid.CurrentCell = ModGrid.Rows[0].Cells[SelectableColumnIndex()];

And the updates are marked by looping over ModGrid.Rows, which only includes visible rows.

Changes

• Now we check that ModGrid.Rows isn't empty before trying to access a row
• Now we look for updates in mainModList.full_list_of_mod_rows.Values instead of just the visible rows

Fixes #3078.

[DasSkelett]

Neat, works fine (and even reduces code size by one line!)