wmaker: dynamic vim-like window marking feature

This patch is adding vim-like window marking, like in i3.
A window can be dynamically assigned a mark label.
Then a marked window can be pulled or jumped to.
Or the current focused window can be swapped with a marked window.

The mark label appears in the Window List in between the window
title and the workspace name.

Those new options in WindowMaker conf file are used to control
the actions: MarkSetKey, MarkUnsetKey, MarkBringKey, MarkJumpKey
and MarkSwapKey.

Those actions are set to None by default.

src/window.h

@@ -299,6 +299,7 @@ typedef struct WWindow {
 	int icon_w, icon_h;
 	RImage *net_icon_image;			/* Window Image */
 	Atom type;
+	char *mark_key_label;		/* Vim-like Window Marking */
 } WWindow;
 
 #define HAS_TITLEBAR(w)		(!(WFLAGP((w), no_titlebar) || (w)->flags.fullscreen))
@@ -323,6 +324,7 @@ typedef struct WSavedState {
     unsigned int w;
     unsigned int h;
     unsigned window_shortcuts; /* mask like 1<<shortcut_number */
+	char *mark_key;	/* serialised mark key label */
 } WSavedState;
 
 typedef struct WWindowState {
@@ -407,6 +409,10 @@ Bool wWindowObscuresWindow(WWindow *wwin, WWindow *obscured);
 
 void wWindowSetOmnipresent(WWindow *wwin, Bool flag);
 
+/* Vim-like window marking management */
+void wWindowSetMark(WWindow *wwin, const char *label);
+void wWindowUnsetMark(WWindow *wwin);
+
 #ifdef XKB_BUTTON_HINT
 void wWindowGetLanguageLabel(int group_index, char *label);
 #endif