Palette editor improvements

[AliceLR]

Add some improvements to the palette editor. Some of these were requested by Graham, and the rest I thought would be useful improvements for SMZX mode 3 editing and/or discussed with Lancer-X.

• Display the stored color in the editor help menu.
• SMZX modes 2/3: allow the user to store (F5) the four colors referenced by the current subpalette and place these at the current subpalette (F6).
• SMZX mode 3: allow the user to store the current subpalette indices (F7) and place the stored subpalette indices at the current subpalette (F8). Also, display the stored subpalette indices in the editor help menu.
• SMZX mode 3: display subpalette cursors over the four colors in the current subpalette. The key to toggle the subpalette cursors has been changed to Insert, since mode 3 uses Space for something else.
• Allow the user to switch between the game palette and temporary secondary working palette (Tab).
• Import (Alt+I) and export (Alt+X) from the palette editor.
• More testing (asan would be nice).
• User testing: mzxgit-x64.zip (if you need some SMZX palettes to test with, here's some astral uploaded a while back and here's a .palidx file by Graham that gives SMZX mode 3 the same mapping as mode 2).