Reduce X11 environment sensitivity in ParaView GUI tests

ParaView's UI tests are quite sensitive to environmental settings such as default font choice, size, theme selection, etc. Potential solutions:

Force a theme

We could force a specific theme in Qt for our tests (we would need to decide what this is) by some mechanism. Preferably in the XML of the test itself so that it is explicit.

Tasks

• Select a theme per platform
• Bail in the test if it can't be loaded or selected

Pros

• One-stop-shop fix

Cons

• Test recording should be done with this theme which might mean a restart of ParaView is needed
• This theme might differ from platform to platform even so

Reduce environmental sensitivity

This is my preferred solution, but is a longer road. Basically, we remove any pixel information during recording of events or the like (except for viewport coordinates since that is under way more control already) and instead rely on other way of accessing information. The biggest culprit right now is pqPipelineBrowser, though there are probably others.

Tasks

• Fix pqPipelineBrowser's recording mechanisms to handle QModelIndex-based recording. This likely means making pqFlatTreeView inherit from QAbstractItemView which requires some more APIs to be implemented.
• Re-record tests to use the new recording format

Pros

• Robust against Qt/GTK theme changes
• Still platform-independent
• Tests recorded on one platform should work everywhere automatically

Cons

• Implementing QAbstractItemView doesn't seem trivial (need Qt experience at least)
• Tests need re-recorded

Cc: @utkarsh.ayachit