osd: add window-switcher custom field (#1670)

Add custom field with subset of printf style formatting
to replace the original field formats.

Example:

    <windowSwitcher preview="no" outlines="no" allWorkspaces="yes">
      <fields>
        <field content="custom" format="foobar %b %3s %-10o %-20W %-10i%t" width="100%" />
      </fields>
    </windowSwitcher>

Mono space font recommended. May need OSD width adjusted

Co-authored-by: @Consolatis (based on work done by them)

docs/labwc-config.5.scd

@@ -243,6 +243,30 @@ this is for compatibility with Openbox.
 
 		- *output* Show output id, if more than one output detected
 
+		- *custom* A printf style config that can replace all the above
+		  fields are:
+			- 'B' - shell type, values [xwayland|xdg-shell]
+			- 'b' - shell type (short form), values [X|W]
+			- 'S' - state of window, values [M|m|F] (3 spaces allocated)
+			        (maximized, minimized, fullscreen)
+			- 's' - state of window (short form), values [M|m|F] (1 space)
+			- 'I' - wm-class/app-id
+			- 'i' - wm-class/app-id trimmed, remove "org." if available
+			- 'W' - workspace name
+			- 'w' - workspace name (if more than 1 ws configured)
+			- 'O' - output name
+			- 'o' - output name (show if more than 1 monitor active)
+			- 'T' - title of window
+			- 't' - title of window (if different than wm-class/app-id)
+		  Recommend using with a mono space font, to keep alignment.
+		- *custom - subset of printf options allowed -- man 3 printf*
+			- random text may be inserted
+			- field length, example "%10" use 10 spaces, even if text uses less
+			- left jusify text, example "%-"
+			- right justify text, example "%" instead of "%-"
+			- example, %-10 would left justify and make room for 10 charaters
+		- Only one custom format allowed now. Future enhancements may allow more than one.
+
 	*width* defines the width of the field expressed as a percentage of
 	the overall window switcher width. The "%" character is required.