Replace Device::getDPI()

[HeikoKlare]

The Device::getDPI() method does have a reasonable meaning when used while monitor-specific scaling is activated. In order to reflect this, the following needs to be done:

1. Known usages of that method shall be replaced (at least in Eclipse Platform, in GEF Classic). Depending on the context, maybe the DPI is not even the desired information (or rather something like the zoom of a window), or a potential replacement might be the usage of per-monitor DPI (with something like OS#getDPIForMonitor()).
   • Replace usages of getDPI in Platform repository #174
2. The method shall be deprecated. Depending on the needs identified in the first point, a replacement may be provided or at least information about way to replace calls to the methods as applied in the first point shall be added to the deprecation notice.

This is a follow-up to:

• Analyse Display::getDPI contract #155