Fix for unusual [theme] font_rgb attribute behavior with unit status labels

Internally, the font_rgb="RR,GG,BB" specification for theme elements is
converted and packed into an unsigned 32-bits integer using the
0x00RRGGBB layout many SDL functions expect. However, for displaying the
unit status report we use ttext (the Pango+Cairo text rendering type),
which expects a different layout: 0xRRGGBBAA. This caused unit status
labels to lack a red channel value and use the blue channel as alpha.

Since the theme UI implementation ignores the alpha value by design (it
only expects three values), force alpha to be 0xFF and shift the color
value to the left so it matches ttext's expectations.

If I had to guess where this bug came from, I'd expect it was
accidentally introduced when converting the theme sidebar to use
Pango+Cairo since the font_rgb attribute seems to have gone unused and
undocumented for years.
Make is_path_sep() a public function and document it
gui2/twml_error: Use filesystem.cpp's is_path_sep() instead of our own thing
Add a white space between the side flag and number in the sidebar.
Add a testcase for units with level >9.
Adjust space for the unit side flag and colors for attributes.
Abuse the hotkey scope system to show undiscovered units in the editor.
Fix bug #21542: Observers icon on the top bar overlaps the Menu button
fix bug #21485: Time Schedule Menu button crashes game
Don't show music tracks which can't be found and spit out a warning.
DM S12: Fix typo 'enemey' -> 'enemy', spotted by wmllint
UtBS S12: Fixed broken Divine Incarnation unit type due to missing [base_unit]

Spotted by wmllint, introduced by commit d46b7e29a02156cf633d42bd6e2c7e35d4ad50e4.
EI S11: Exempt a bat's line from wmllint spellchecking
Make all labels visible in the editor.

I don't know why the earlier logic was so convoluted. Someone tried
to mash so many conditions in one line that apparently I missed the
intent earlier. I'm no longer trying to match the earlier logic, but
instead just trying to get the conditions right.

I really don't know how this will interact with the map editor's
label tool, because that tool is disabled for me and the screen
gives me no indication of how to enable it. But this does appear
to fix bug #21732 without messing up multiplayer.
Revert "apply a blindfold during skip replays"

This reverts commit b0023b7e9c1673d452ba1dad119659df929b7bae.
The earlier commit was supposed to be on a testing branch...
my apologies, this change would not be appropriate during
feature freeze.