bind control-shift-u to unicode-input, move urls to o

Having a keybinding to invoke arbitrary unicode characters is very
useful. It's often used as a method of last resort to communicate with
people outside of your main language. For example, if you want to type
the last letter of my real name, you can invoke the latin-1 character
0xe9 or unicode 0x00e9.

You can also use this to type special characters, for example, unicode
U+1F4A9 is of course, the infamous PILE OF POO, which is sure to
produce million laughs everywhere you go.

In foot, there's no keybinding by default to invoke the very useful
unicode-input command. There is no "standard" (as in "ISO") keybinding
this either. But there *is* a de-facto standard currently deployed
by *both* GTK and Qt (a rare feat) *and* Chrome OS (an even rarer
feat) and it's control-shift-u.

Alternatives include Control-x 8 (emacs), Control V u (vim),
Alt (Windows, LibreOffice), or Option (Mac). I doubt we want to adopt
any of those.

So let's use control-shift-u for this. Unfortunately, it's currently
assigned to show-urls-launch, which is unfortunate, but
insurmountable. We can reassign this keybinding elsewhere. I have
picked control-shift-o in my configuration, because "o" is a good
mnemonic for "open URLs". Others have suggested "m" instead.

Closes: #1183

CHANGELOG.md

@@ -60,6 +60,9 @@
 
 ### Changed
 
+* `Control+Shift+u` now bound to `unicode-input` to follow the
+  convention established in GTK and Qt, `show-urls-launch` now bound
+  to `Control+Shift+o` ([#1183][1183])
 * Default color theme from a variant of the Zenburn theme, to a
   variant of the Solarized dark theme.
 * Default `pad` from 2x2 to 0x0 (i.e. no padding at all).
@@ -74,6 +77,7 @@
 
 [1166]: https://codeberg.org/dnkl/foot/issues/1166
 [1179]: https://codeberg.org/dnkl/foot/issues/1179
+[1183]: https://codeberg.org/dnkl/foot/issues/1183
 [1215]: https://codeberg.org/dnkl/foot/pulls/1215