Easy things for Android port

[Po Lu]

The Android port, found in feature/android, is now useful for editing,
given that you have a mouse and keyboard.

If someone wants to do easy things for the Android port, they would be:

  - make dired.c understand the `/assets' special directory.

  - make drawing faster by avoiding the extra canvas save and restore on
    each graphics op, only doing that if clip rectangles really change.

  - add support for touch screen events, like in xterm.c, and the
    on-screen keyboard.

  - add support for the TrueType Container format to sfnt.c and
    sfntfont.c.  This is easy because TTC files are just TrueType files
    with multiple table directories, each representing a single font.

  - add support for window restacking and geometry operations
    (i.e. raise-frame, android_reparent_window, et cetera.)