Add view properties

[ammen99]

This PR adds a more streamlined way to attach custom data (integers, strings, etc.) to views under a given key - properties. They are meant to be used for setting metadata which is unrelated to the 'main' view state (geometry, minimized/tiled/fullscreen state, etc.), as well as custom properties that various plugins can use.

Planned properties for this first PR:

• startup-x and startup-y to replace is_positioned in the view-mapped signal. This way Xwayland can indicate it has a custom position for the views and the place plugin should not move them. Similar could be set via IPC or any custom plugin.
• A way to create IPC scripts which delay the map transaction, allowing IPC clients to set custom properties (and also set initial geometry, etc.)
• A way to set properties via window-rules or similar (will be IPC only for now)
• open-animation-type, open-animation-duration, close-animation, close-animation-duration for the animate plugin (fixes Per-window animations & effects / Window rules for animations & effects #1832)