Make gaps implementation consistent with i3-gaps

This changes our gaps implementation to behave like i3-gaps.

Our previous implementation allowed you to set gaps on a per container
basis. This isn't supported by i3-gaps and doesn't seem to have a
practical use case. The gaps_outer and gaps_inner properties on
containers are now removed as they just read the gaps_inner from the
workspace.

`gaps inner|outer <px>` no longer changes the gaps for all workspaces.
It only sets defaults for new workspaces.

`gaps inner|outer current|workspace|all set|plus|minus <px>` is now
runtime only, and the workspace option is now removed. `current` now
sets gaps for the current workspace as opposed to the current container.

`workspace <ws> gaps inner|outer <px>` is now implemented. This sets
defaults for a workspace.

This also fixes a bug where changing the layout of a split container
from linear to tabbed would cause gaps to not be applied to it until you
switch to another workspace and back.

include/sway/config.h

@@ -173,6 +173,8 @@ struct output_config {
 struct workspace_config {
 	char *workspace;
 	char *output;
+	int gaps_inner;
+	int gaps_outer;
 };
 
 struct bar_config {