Also added _NET_WM_ICON_NAME.

Features:

- Allow input methods swap with hotkey (E.g. left ctrl + left shift).
- Over-the-spot pre-editing style, pre-edit data placed over insertion point.
- Restart IME without segmentation fault.

TODO:

- Automatically pickup IME if st started before IME

When possible, declare functions/variables static and move struct
definitions out of headers.  In order to allow utf8decode to become
internal, use codepoint for DECSCUSR extension directly.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

Gradually reducing x.c dependency on Term object.  Old and new cursor
glyph/position are passed to xdrawcursor.  (There may be an opportunity
to refactor further if we can unify "clear old cursor" and "draw new
cursor" functionality.)

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

Introduces three functions to encapsulate X-specific behavior:
 * xdrawline: draws a portion of a single line (used by drawregion)
 * xbegindraw: called to prepare for drawing (will be useful for e.g.
   Wayland) and returns true if drawing should happen
 * xfinishdraw: called to finish drawing (used by draw)

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

Moves the mode bits used by x.c from Term to TermWindow, absorbing
UI/input-related mode bits (visible/focused/numlock) along the way.

This is gradually reducing external references to Term.  Since
TermWindow is already internal to x.c, we add xsetmode() to allow st to
modify window bits in accordance with escape sequences.

IS_SET() is redefined accordingly (term.mode in st.c, win.mode in x.c).

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

None of the X-related includes are needed any longer.  In addition, move
the X modifier defines into x.c, as they are not used outside.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

This is an X type and should be internal to x.c.

The selcopy() function was a single line and only used in one place, so
it was inlined to reduce LOC.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>