Change: replace per-AI "start_date" with a global "competitors_interval" (#10653)

The per-AI "start_date" is a lot of custom code, and was rarely
used in the way it was meant.

While at it, also ported this part over to the new timer system.

src/company_type.h

@@ -42,6 +42,9 @@ static const uint MAX_LENGTH_COMPANY_NAME_CHARS   = 32; ///< The maximum length
 static const uint MAX_HISTORY_QUARTERS            = 24; ///< The maximum number of quarters kept as performance's history
 static const uint MAX_COMPANY_SHARE_OWNERS        =  4; ///< The maximum number of shares of a company that can be owned by another company.
 
+static const uint MIN_COMPETITORS_INTERVAL = 0;   ///< The minimum interval (in minutes) between competitors.
+static const uint MAX_COMPETITORS_INTERVAL = 500; ///< The maximum interval (in minutes) between competitors.
+
 /** Define basic enum properties */
 template <> struct EnumPropsT<Owner> : MakeEnumPropsT<Owner, byte, OWNER_BEGIN, OWNER_END, INVALID_OWNER> {};