Correct spelling in comments and log messages.
Open Source window: Double-click/Enter opens file

    * src/persp/dbgperspective/nmv-open-file-dialog.cc
    (on_chooser_file_activated): New function invokes action of okbutton
    when the Enter key is pressed or a file is double-clicked. (Before
    changing nmv-open-file-dialog.c I set "has_default" for the okbutton
    in openfiledialog.ui: Enter key was handled, but double-click was
    not.)
Polish code

    * src/main.cc: Update as per coding_style.txt section "Null, false".
    * src/workbench/nmv-workbench.cc: Update as per coding_style.txt
      section "Null, false".

    * src/persp/dbgperspective/nmv-file-list.cc: Delete incorrect
      comment.
    * src/dbgengine/nmv-dbg-common.cc: Delete redundant comment.
    (on_files_listed_signal): Delete code to silence unused function
    parameter compiler warning - use unnamed function parameter instead.

    * src/persp/dbgperspective/nmv-open-file-dialog.cc: Prefix names of
    functions returning bool with "is"/"are".
    Replace code such as "if (a_bool) do_it (true); else do_it(false);"
    with "do_it (a_bool);"
    * src/persp/dbgperspective/nmv-dbg-perspective.cc:
    (DBGPerspective::ask_user_to_select_file): delete unused function.
    (DBGPerspective::append_source_editor): construct message object
    where it's used.
    (DBGPerspective::execute_program): construct objects where they're
    used.
    (DBGPerspective::append_visual_breakpoint): Replace code like
     "if (a_bool) return true; else return false;"
    with
     "return a_bool;"
    Update as per coding_style.txt section "Null, false".
    * tests/test-var-list.cc: Miscellaneous style fixes.
    * common/nmv-parsing-utils.cc: Simplify functions returning bool.
Don't list "<built-in>" in Open Source File window

    * src/dbgengine/nmv-gdbmi-parser.cc (GDBMIParser::parse_file_list):
    filter out filename ending with "<built-in>" as it does not name a
    real file.
    * src/common/nmv-ustring.cc (UString::ends_with): New function.
    * src/common/nmv-ustring.h (UString::ends_with): New function.