better styles/resources fallback support

[Philippe Normand]

With current upicek:

1. Create a plugin containing a new styles.conf
2. Use it as frontend theme
3. Elisa doesn't start.

Because one of the poblesec resources "module_name" doesn't exist as a valid python module path (it's one of the control ribbon glyphes). And because resources lookup is kinda based on that, the Theme fails to load the resources.conf located at that wrong location.

Needs further investigation. Maybe merge fallback themes all together aside from the default theme. This solution would also be more reliable speed-wise.

[Philippe Normand (philn)]

Merging fallback themes doesn't work. The first non-existing paths I see before Elisa crashes are:

- elisa.plugins.poblesec.player.back_button
- elisa.plugins.poblesec.player.ribbon

So, either we fix the resources paths or every time a Theme is created we merge it with the default Theme instance

[Philippe Normand (philn)]

Sorry, I was wrong. Merging fallback theme in current theme instance works but has the side effect that the default theme can potentially be "polluted" with properties of other styles/resources. This breaks the Theme override system. See also Bug 257310