With this, there is plenty of palces where the DPI can be set before
those signals are sent. This allows wallpaper with the proper DPI to
work with screens created using `fake_add`. In turn, this will allow
screen rules to control the DPI. In "the past", the DPI used for those
handler was the native DPI of the screen with no opportunity to change
it before hand.
Add `dpi.lua` to config.ld even if it isn't added yet. This is
because the way the test run has it cached in the build dir. A full
rebuild would take too long and timeout on travis for semi-large PRs.
Emmanuel Lepage Vallee