DefaultPluginManager doesn't allow Plugin definitions from Themes

Problem/Motivation

There are some plugins that are related to Front End, like Breakpoints and Layout_plugin. This plugins should allow themes to add definitions when needed.

The findDefinitions method on the DefaultPluuginManager check for the existance of the provider as a method, this leads the method to unset any definition find inside themes.

To workaround this the Breakpoint plugin and Layout_plugin are taking the path of duplicating the findDefinitions, pretty much the same code but adding an extra check for themes; this leads to duplicated code and in general not really good practice.

Proposed resolution

There are some ways to do this:
1. in findDefinitions: check if the pluginmanager has a themehandler and then add a check for themeexists().
2. Add a method ProviderExists(provider) that can be overridden by the pluginmanager and returns true if the theme or module exist, just the module by default. (Based on Tim Plunkett's idea)
3. just simply always allow definitions if provider is an existing theme.

Remaining tasks

a. Decide on a method.
b. Do the simple fix for that, I pesonally like the second one and will provide a patch soon.

User interface changes

None

API changes

None