UI: Consider glBitmap and glEnd drawcalls when using the "Prev DC" and "Next DC" buttons
UI: Update trim dialog so that it does not accept invalid input.
* Clicking "OK" with invalid input would not set the member variables, but would return as accepted, resulting in a failed trim.
* Improved error messages if trim_frame or trim_len are invalid, and also now prompt the user if the entered trim_file already exists.
UI: Add a settings file
* When vogleditor is started, the settings file will be loaded / saved so that it contains all the latest settings.
* The JSON settings file is stored in $XDG_CONFIG_HOME/vogleditor/ or $HOME/.config/vogleditor/ and is named vogleditor_settings.json.
* Before manually editing the file, vogleditor should be closed; after editing the file, reopen vogleditor to read in the latest changes.
* The file is automatically saved when vogleditor is closed, so manual edits while the file is open will cause the changes to be overwritten.
* Current settings include:
  * Window position and dimensions (defaults to 0,0 and 1024x768)
  * The "trim_large_trace_prompt_size" (defaults to 200) which is the minimum number of frames a trace file must contain in order for the trim trace prompt to be displayed when loading a trace file.
Merge branch 'master' of https://github.com/ValveSoftware/vogl