Show update and issue informations

[leukipp]

With the release of cortile v2.3.0 , an online version checker and update indicator were introduced (see #23).

In v2.5.1, this feature has been further enhanced to request the following information from GitHub:

• Fetch the latest release and display an update available hint if the current running version is outdated.
• Fetch all pinned issues on startup and show them in the system tray menu.

As a result, two requests to the Github API will be made when the application starts.

The hint symbol will disappear once you have reviewed all the open information entries. This functionality will only work if caching has not been globally disabled.

[leukipp]

In addition to the information that a new release is available, there is now also an Updates section in the systray menu, which immediately replaces the running cortile instance with the latest version available on github when clicked.
However, this only works if the path of the currently executed binary file is writable under the user who is running cortile.

Cortile releases adhere to the semver specification. Consequently, the update feature is only available for releases that include backward-compatible bug fixes (patches).

For minor or major releases, you may need to perform some backward-compatible steps (e.g. add/update entries in the config file) before applying the update. In those cases, the cortile update must still be carried out manually by downloading and replacing the existing binary file.