Feature: Github version check added to the QT wallet.

[Germardies]

A check has been added when starting the QT wallet.

1. Check: Is Github accessible?
2. Check: Version comparison between local wallet and Github.

The check is performed every time the QT wallet is started.
A message indicating that a new version is available appears as soon as a difference is detected.

Tested in Ubuntu.

[JSanchezFDZ]

Reviewed the update checker. The class is self-contained and fails silently on
network errors, which is good. A few things worth addressing before merge:

1. Opt-out / privacy. It runs on every GUI start with an unconditional
   outbound request to api.github.com. Please gate it behind a setting (an
   Options checkbox and/or -checkforupdates, defaulting per project policy) so
   privacy-sensitive, offline, or custom builds can turn it off. Bitcoin Core
   deliberately ships without a built-in update check for this reason.

2. Version parsing misses the build component. isNewer() matches only
   (\d+)\.(\d+)\.(\d+) and the local string is built from MAJOR.MINOR.REVISION,
   dropping CLIENT_VERSION_BUILD. Raptoreum release tags are 4-part (the current
   release is v2.0.04.01), so a build-level bump (the .01) is never detected
   and 2.0.04.01 compares equal to a local 2.0.04. Please include the 4th
   component on both sides and match four groups.

3. Lexicographic fallback. return remote > local is only reached when the
   regex fails, but string comparison is wrong for multi-digit parts ("10" < "9").
   Worth hardening or removing.

4. Modal dialog during startup. The notification is a modal QMessageBox::exec()
   fired right after createWindow(), before the init/shutdown thread starts.
   Consider showing it non-modally once the main window is ready so it cannot
   interfere with the splash/startup sequence.

5. Minor: updatechecker.cpp is missing a trailing newline.