[Enhancement] Change the mouse pointer to cross type move one when hovering the selected area

[SoftVision-CosminMuntean]

[Notes]:

• We should add this change because it's not entirely clear that the selection can be moved from any point of it.

[Affected versions]:

• Firefox 48.0.1 and up
• Mozilla Pag eShot 0.1.201610172213

[Affected Platforms]:

• All Windows
• All Mac OS
• All Linux

[Prerequisites]:

• Latest Page Shot add-on (v0.1.201610172213) is installed on a clean profile.

[Steps to reproduce]:
1 Open the browser with the profile from prerequisite and navigate to any page.
2. Click the Page Shot icon from the browser toolbar and perform a selection.
3. Hover the selection and observe the cursor pointer.

[Expected result]:

• The cursor pointer changes into a "move" cross type pointer showing that the selected area can be moved around from any point.

[Actual result]:

• The cursor pointer remains unchanged.