rename: replace "compton" in the codebase

leftovers:

1) config file path. Has to implement compatibility functionalities before
  we can change it.

2) links in man pages. Has to migrate the repo first.

3) _COMPTON_SHADOW, it has become a defacto standard, so we have to keep
   supporting it.

4) dbus names, undecided whether we should/could change it.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

man/picom-trans.1.asciidoc

@@ -0,0 +1,84 @@
+picom-trans(1)
+================
+:doctype:     manpage
+:man source:  picom
+:man version: {picom-version}
+:man manual:  User Commands
+
+NAME
+----
+picom-trans - an opacity setter tool
+
+SYNOPSIS
+--------
+
+*picom-trans* [-w 'WINDOW_ID'] [-n 'WINDOW_NAME'] [-c] [-s] 'OPACITY'
+
+DESCRIPTION
+-----------
+
+*picom-trans* is a bash script that sets '_NET_WM_WINDOW_OPACITY' attribute of a window using standard X11 command-line utilities, including *xprop*(1) and *xwininfo*(1). It is similar to *transset*(1) or *transset-df*(1).
+
+OPTIONS
+-------
+*-w* 'WINDOW_ID'::
+Specify the window id of the target window.
+
+*-n* 'WINDOW_NAME'::
+Specify and try to match a window name.
+
+*-c*::
+Specify the currently active window as target. Only works if EWMH '_NET_ACTIVE_WINDOW' property exists on root window.
+
+*-s*::
+Select target window with mouse cursor. This is the default if no window has been specified.
+
+*-o* 'OPACITY'::
+Specify the new opacity value for the window. This value can be anywhere from 1-100. If it is prefixed with a plus or minus (+/-), this will increment or decrement from the target window's current opacity instead. 
+
+EXAMPLES
+--------
+
+* Set the opacity of the window with specific window ID to 75%:
++
+------------
+picom-trans -w "$WINDOWID" 75
+------------
+
+* Set the opacity of the window with the name "urxvt" to 75%:
++
+------------
+picom-trans -n "urxvt" 75
+------------
+
+* Set current window to opacity of 75%:
++
+------------
+picom-trans -c 75
+------------
+
+* Select target window and set opacity to 75%:
++
+------------
+picom-trans -s 75
+------------
+
+* Increment opacity of current active window by 5%:
++
+------------
+picom-trans -c +5
+------------
+
+* Decrement opacity of current active window by 5%:
++
+------------
+picom-trans -c -- -5
+------------
+
+BUGS
+----
+Please submit bug reports to <https://github.com/yshui/compton>.
+
+SEE ALSO
+--------
+link:picom.1.html[*picom*(1)], *xprop*(1), *xwininfo*(1)