feature: add debuginfod options and solve #130 partially.

Signed-off-by: varun-r-mallya varunrmallya@gmail.com

Once completed, should solve #130 partially. Status:

• Toggle support for debuginfod. Some people may want this always off.
• Add support to specify alternate $DEBUGINFOD_URLS to use with libdebuginfod. This can be useful in IoT/server scenarios which do not match the local machine.
• Allow specifying a list of debug file directories (useful for IoT where you have debuginfo files on workstation) (Will be completed in another MR.)
• GSettings can be used to also persist callgraph toggles across runs of Sysprof
• Persist state using JSON to a user data-dir and load from that.

Tests have not been updated yet for debuginfod. Waiting for the first question below to be resolved.

Questions:

• $DEBUGINFOD_URLS is an environment variable right ? Is there a function to set it or should I write my own to set it to the environment ? (Should I be reusing the ones from the "add variable" environment variable setting in the "application" tab on the greeter ?). If we are anyway setting an environment variable, can't the user just use the add variable section in the application tab ?
• Also, do you like this UI layout ?