project: Add ability to exclude folders

Sometimes we have folder around in the project that don't need to be shown in the project tree. For git projects, you can just gitignore them but for non-git folder based projects there is no way to do this.

With this commit, you can now specify a "exclude_folder_patterns" in the .kateproject file like:

{
   "exclude_folder_patterns": ["node_modules", "blah"]
}

and any file/folder that matches this pattern is ignored. Patterns are normal regexp patterns.

Atm excluding is only possible via .kateproject file. In future, we can provide two more ways:

• Via config, a global "exclude" list that is always honored
• By right click - "Exclude Folder" => results in creation of .kateproject file at the project root