gui.frametool: use enum, not str, for performance

The issue was not so much string comparisons, but a ton of redundant
calls to gettext. The way things are set up here are questionable
overall, and should be rewritten completely, but that is more about
code quality than performance.

gui/framewindow.py

@@ -499,11 +499,9 @@ def _init_ui(self):
         color_button.connect("color-set", self._color_set_cb)
 
         unit_combobox = Gtk.ComboBoxText()
-        for unit in UnitAdjustment.CONVERT_UNITS.keys():
-            unit_combobox.append_text(unit)
-        for i, key in enumerate(UnitAdjustment.CONVERT_UNITS):
-            if key == _('px'):
-                unit_combobox.set_active(i)
+        for unit in sorted(UnitAdjustment.CONVERT_UNITS.keys()):
+            unit_combobox.append_text(_Unit.STRINGS[unit])
+        unit_combobox.set_active(_Unit.PX)
         unit_combobox.connect('changed', self.on_unit_changed)
         unit_combobox.set_hexpand(False)
         unit_combobox.set_vexpand(False)
@@ -592,14 +590,6 @@ def _new_key_label(cls, text):
         label.set_margin_end(6)
         return label
 
-    def get_unit_text(self):
-        combobox = self._unit_combobox
-        model = combobox.get_model()
-        active = combobox.get_active()
-        if active < 0:
-            return None
-        return model[active][0]
-
     def crop_frame_cb(self, button, command):
         model = self.app.doc.model
         if command == 'CropFrameToLayer':
@@ -615,7 +605,7 @@ def _color_set_cb(self, colorbutton):
         self.app.doc.tdw.queue_draw()
 
     def on_unit_changed(self, unit_combobox):
-        active_unit = self.get_unit_text()
+        active_unit = unit_combobox.get_active()
         self.width_adj.set_unit(active_unit)
         self.height_adj.set_unit(active_unit)
 
@@ -825,14 +815,28 @@ def paint(self, cr):
         editmode.remove_button_pos = button_pos
 
 
+class _Unit:
+    PX = 0
+    IN = 1
+    CM = 2
+    MM = 3
+
+    STRINGS = {
+        PX: _('px'),
+        IN: _('in'),
+        CM: _('cm'),
+        MM: _('mm'),
+    }
+
+
 class UnitAdjustment(Gtk.Adjustment):
 
     CONVERT_UNITS = {
         # {unit: (conv_factor, upper, lower, step_incr, page_incr, digits)}
-        _('px'): (0.0, 32000, 1, 1, 128, 0),
-        _('inch'): (1.0, 200, 0.01, 0.01, 1, 2),
-        _('cm'): (2.54, 500, 0.1, 0.1, 1, 1),
-        _('mm'): (25.4, 5000, 1, 1, 10, 0),
+        _Unit.PX: (0.0, 32000, 1, 1, 128, 0),
+        _Unit.IN: (1.0, 200, 0.01, 0.01, 1, 2),
+        _Unit.CM: (2.54, 500, 0.1, 0.1, 1, 1),
+        _Unit.MM: (25.4, 5000, 1, 1, 10, 0),
     }
 
     def __init__(self, value=0, lower=0, upper=0, step_increment=0,
@@ -846,8 +850,8 @@ def __init__(self, value=0, lower=0, upper=0, step_increment=0,
         )
         self.px_value = value
         self.unit_value = value
-        self.active_unit = _('px')
-        self.old_unit = _('px')
+        self.active_unit = _Unit.PX
+        self.old_unit = _Unit.PX
         self.dpi = dpi
 
     def set_spin_button(self, button):
@@ -900,11 +904,11 @@ def convert(self, value, unit_from, unit_to):
         return uvalue
 
     def convert_to_px(self, value, unit):
-        if unit == _('px'):
+        if unit == _Unit.PX:
             return value
         return value / UnitAdjustment.CONVERT_UNITS[unit][0] * self.dpi
 
     def convert_to_unit(self, px, unit):
-        if unit == _('px'):
+        if unit == _Unit.PX:
             return px
         return px * UnitAdjustment.CONVERT_UNITS[unit][0] / self.dpi