Support for CH- / CH+ keys for easier channel navigation

[sguinetti]

Is your feature request related to a problem? Please describe.
On a traditional TV, to change channels, you have to press the CH- (previous channel) and CH+ (next channel) buttons to switch from one channel to another. It is very simple. Taking advantage of the new feature, channel navigation on the M3U app can be accessed from the GUI, which is useful on touch screens. But it is not accessible with the traditional remote control buttons, which becomes a wasted option.

Describe the solution you'd like
Enable CH- and CH+ buttons to speed up channel navigation.

Describe alternatives you've considered
In the Smarttube application, streams / channels are switched with the CH- and CH+ keys without the need to go to the menu and press the respective commands from D-Pad instead.

Additional context
Note 1: Smarttube has a keymapping option to set shortcuts from TV remote buttons.

Note 2: In addition to the CH buttons, other remote control buttons such as Subtitle setting can be mapped, depending on the specific brand. See https://developer.android.com/reference/android/view/KeyEvent for more context and https://helpguide.sony.net/tv/ggar1/v1/en-142/01-07-02.html for a example of TV remote control.