gatom (and upcoming popup menu) should be resizable by dragging mouse

Currently gatom can't be resized by click-dragging on the right side of the box. But message and object boxes can.

This is because:

• the conditional in line 3354 of g_editor.c requires the object to have a widget behavior equal to &text_widgetbehavior, but gatom has &gatom_widgetbehavior
• the conditional in line 5220 of g_editor.c makes the same requirement

A quick fix could probably be to just check the class name for gensym("gatom") in that conditional. Then I could easily add the upcoming popup menu classname there, too-- its widgetbehavior is close enough to gatom that it should work the same.

It appears that gatom does the "right thing" when click-dragging the width. The new width is properly reflected in the dialog, and it gets saved correctly, too. The complication here is that the click-dragging adds a , f $width suffix to the object's Pd file line, which is in addition to gatom's width argument. But Pd appears to already be keeping these two in sync.