Use list_find in more places and refactor/fix workspace prev_next functions

The original purpose of this commit is to replace some for loops with
list_find. But while doing this I found the workspace_prev_next_impl
functions to be difficult to read and also contained a bug, so I
refactored them and fixed the bug.

To reproduce the bug:

* Have two outputs, where the left output has workspaces 1, 2, 3 and the
right output has workspaces 4, 5, 6. Make workspace 2 focused_inactive
and workspace 4 focused.
* Run `workspace prev`.
* Previously it would visit the left output, then apply `workspace prev`
to workspace 2, which focuses workspace 1.
* Now it will focus the rightmost workspace on the left output
(workspace 3).

The refactoring I made to the workspace functions are:

* Added the static keyword.
* They now accept an int dir rather than bool, to avoid an unnecessary
conversion.
* Rather than preparing start and end variables for the purpose of
iterating, just iterate everything.
* Replace for loops with list_find.
* Don't call workspace_output_prev_next_impl (this fixes the bug).

common/list.c

@@ -77,7 +77,7 @@ int list_seq_find(list_t *list, int compare(const void *item, const void *data),
 	return -1;
 }
 
-int list_find(list_t *list, void *item) {
+int list_find(list_t *list, const void *item) {
 	for (int i = 0; i < list->length; i++) {
 		if (list->items[i] == item) {
 			return i;