Implement nested sections

[Sidnioulz]

Some compositors implement modes or multiple environments and would like different policies for such environments.

Mandatory design/code changes:

• add way to change compositor name or much better to provide/retrieve it at permission granting time
• explore methods to prevent race conditions on compositor identity when granting permissions
• explore methods to prevent delegating statefulness management to compositors to avoid errors; find a clear method to guarantee the validity of an access decision when it is received by the compositor

Two appraches at least:

1. get compositors to advertise new names and use a section per mode

• No added complexity to policy reading
• But added length: harder to author meaningful policies (must copy/paste additions in each section) and even more so to edit them programmatically (are sections identical by coincidence or because they should be sync'd?).
• Trivial code changes

Changes to codebase:
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2. implement subsections

• More compact, faster to edit for experts
• Less readability within file
• Might require editing the parser substantially
• Added complexity throughout whole backend, might need to change get_permission to expose contextual cues on compositor identity

Changes to codebase:

• add a subsection syntax for the files
• make parser understand subsections
• rework weston_config_get_section* functions