Add option for configurable maintenance schedules

[RealDekkia]

This PR Adds "maintenance Schedule" as another value-type an inventory-field can have.

The existing imf_value_list field is used to configure the reoccurring maintenance dates as described in the tooltip:

The internal name of the date field the current schedules applies to must be specified with a # symbol. e.g. #LAST_CHECK 
The cycle interval must be written in the same line separated by the |-symbol after the title. e.g. Yearly | 1y 
If no unit is specified, days are assumed. 
Possible units: d: days, w: weeks, m: months, y: years

The configuration can look like the one in the screenshot for example.

The Inventory-table automatically adds a column with the remaining days until the next maintenance after the maintenance-selection row itself:

The Translations for french are machine-generated, so they're probably not perfect.

[MightyMCoder]

On the maintain Itemfields page in fields.php the new field type didn't show up:

[RealDekkia]

On the maintain Itemfields page in fields.php the new field type didn't show up:

Fixed in 4c252b2

[MightyMCoder]

@RealDekkia: Many thanks for your work.
I implemented a separate selectbox to define the corresponding date field so there is no #FIELD_NAME necessary in the value list.