Set EWMH desktop properties on startup.

Calls ewmh_update_current_desktop on startup to set the
_NET_CURRENT_DESKTOP property. Without this change the property only
gets set after the workspaces have been manipulated. Also exclude
hidden workspaces (i.e. those starting with "__" from the workspace
index.

Adds tests for startup and workspace switching.
Doc update: title right click resizes floating windows

This documentation update describes the change implemented in
905440d6d1c6d2a872e3bbb0da4c963bedaf5708
It avoids repetitive usage of the word 'also'.
Bugfix: `tree_split` should not split floating cons

Use the more robust test of `con_is_floating` for testing a container
for being floating.

fixes #1177
fix warnings: unsigned check for >= 0 is always true