Some minor settings changes, mainly to fit Material Design

[Cguy7777]

The main purpose of this PR is to change the preferences strings to match Material Design's recommendations, by changing capitalization and re-wording several strings. It might be a good idea to add a link or note about this to the wiki page about adding preferences to help keep the strings for new preferences in line.

This PR also has a few other minor changes:

• Make the Use Custom Tabs preference dependent on Use internal browser, since it has no effect if that's disabled
• Move Tablet mode (two pane) to the top of the appearance preferences and remove its PreferenceCategory (it was the only one in there)
• Add a summary for Use Tor to remind users that links will not be sent over Tor if they are not using the built-in WebView browser
• Add a summary for Comment header to clarify its purpose

[QuantumBadger]

Merged in, thanks!

edcfd73