Make "Log In" button (etc.) move with start/grid combo visibility.

The "Start at:" combo box and the grid-selector box can be turned on and off
with checkboxes on the Preferences floater. Break the bottom-left layout_panel
on panel_login into several different layout_panels so when these UI elements
disappear, other controls move left to fill the holes. Similarly, when you
turn on either or both of the UI elements in question, the remaining controls
in that general bottom-left area move right to accommodate.
Tweak C++ and non-English panel_login.xml overrides to reflect widgets'
containment in new layout_panel elements.
Discard obsolete "mode_selection_text" and "mode_combo" translations from non-
English panel_login.xml overrides.