Improve loading time

[sebikolon]

When the MedDoser was turned off for a longer time (e.g. three weeks), entries are created within the persistence file Persistence_MedDoser.xml for each missed day.
When the device gets turned on again for the first time, the UI is acting remarkable slower than intended.

The probably reason for this behaviour is

1. The large number of entries must be read from the persistence file (once) and handled in the background (many string comparisons), then saved in a data structure in the RAM.
2. Searching the data structure in the RAM

Probably solution: Store all the data in an SQLite database instead of an XML file, because the columns of the database are already assigned to datatypes.