Remove temp cut buffer commands and cleanup related SourceView code

- Moved cut_to_temp_buffer and paste_temp_buffer commands to plugin
- Remove temp buffer state and timeout logic from SourceView
- Remove associated keybindings (Ctrl+K / Ctrl+U)
- Ignore "buffer" pseudo-path when filtering loaded files
- Ensure view receives focus after DnD file load

user_config/usr/share/newton/code-key-bindings.json

@@ -15,18 +15,12 @@
         "zoom_out": {
             "held": "<Control>minus"
         },
-        "cut_to_temp_buffer": {
-            "held": "<Control>k"
-        },
         "duplicate_line": {
             "held": "<Control>d"
         },
         "go_to": {
             "released": "<Control>g"
         },
-        "paste_temp_buffer": {
-            "held": "<Control>u"
-        },
         "new_file": {
             "released": "<Control>t"
         },