support folding via F1 and clicks in the folding margin

* Set up the folding margin in the currently empty margin column
  after the line number.
  On Gtk, this meant resetting all the marker symbols and their
  foreground/background colors as well as the margin's colors themselves.
  This looks like a bug. It's not necessary on Scinterm, which apparently
  uses the default/linenumber styles by default.
  Perhaps we should try upgrading Scintilla?
* The folding state is considered not to be directly controlled by the
  language (just like the scroll position and zoom level). That's why
  we can directly control it by clicking on the margin column.
* F1 can be used to toggle all folds globally.
* The only support within the C core necessary for folding is to make
  sure that the current line is unfolded after every keypress.
* We might add custom folding commands to the language later on
  (e.g. F+, F-). In this case, the key macros will have to be changed
  of course.

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