Increase size of the accessible elements in the header

Previously, the back button was so small that it was hard to hit it.

Change-Id: I5948d379f47a8d57749acc22bbbb098432afccaf
Reviewed-by: Caroline Chao <caroline.chao@theqtcompany.com>
Call tr() so that lupdate recognizes it.

The old code resulted in complaint from lupdate:
 "weather-app/src/main.cpp:111: tr() cannot be called without context"

Change-Id: I4395d544f43fe1f0a8c55393f378b4d62bdeb73b
Reviewed-by: Caroline Chao <caroline.chao@theqtcompany.com>