[Documentation] Palette preview in TestScreen

[Meridian]

I've added new functionality to test screen (hotkey Ctrl+T while debug: true) to preview all text colors for all palettes and contrasts.

For xcom1-based mods:
 - PAL_GEOSCAPE
 - PAL_BASESCAPE
 - PAL_GRAPHS
 - PAL_UFOPAEDIA
 - PAL_BATTLEPEDIA
 - PAL_BATTLESCAPE, low/high contrast

For xcom2-based mods:
 - PAL_GEOSCAPE
 - PAL_BASESCAPE
 - PAL_GRAPHS
 - PAL_BATTLESCAPE, low/high contrast
 - PAL_BATTLESCAPE_1, low/high contrast
 - PAL_BATTLESCAPE_2, low/high contrast
 - PAL_BATTLESCAPE_3, low/high contrast

High contrast is used on many battlescape GUIs, for example Inventory GUI.

[Meridian]

Added a few more options:

1. Click on LOW or PREVIEW button - shows text in normal/low contrast
2. Click on HIGH button - shows text in high contrast
3. Shift-Click on LOW or PREVIEW button - shows small digits without border
4. Shift-Click on HIGH button - shows small digits with border
5. Ctrl-Click on any button - shows raw palette

Examples attached.