This PR splits the GUI source files from the core source files. The immediate goal is to allow the CLI to require only a minimum number of dynamic libraries. The long term goal is to create an architectural boundary around the core module, in preparation of libkdbx.
* Reduce initial update check notification to 500 ms to prevent inappropriately stealing focus from user
* Add build flag WITH_XC_UPDATECHECK which defaults to ON
* Update checks are resolved every 7 days instead of every time the application is started
* Better checks for beta builds; ignore snapshots
* Increase test cases
* Check on startup (toggleable setting) and manually
* Option to check for pre-releases (eg, 2.4.0-beta1)
* Only included if WITH_XC_NETWORKING is enabled
